Method and system for controlling resistivity in ingots made of compensated feedstock silicon

ABSTRACT

Techniques for controlling resistivity in the formation of a silicon ingot from compensated feedstock silicon material prepares a compensated, upgraded metallurgical silicon feedstock for being melted to form a silicon melt. The compensated, upgraded metallurgical silicon feedstock provides semiconductor predominantly of a single type (p-type or n-type) for which the process assesses the concentrations of boron and phosphorus and adds a predetermined amount of boron, phosphorus, aluminum and/or gallium. The process further melts the silicon feedstock with the boron, phosphorus, aluminum and/or gallium to form a molten silicon solution from which to perform directional solidification and maintains the homogeneity of the resistivity of the silicon throughout the ingot. A balanced amount of phosphorus can be optionally added to the aluminum and/or gallium. Resistivity may also be measured repeatedly during ingot formation, and additional dopant may be added in response, either repeatedly or continuously.

FIELD

The present disclosure relates to methods and systems for use in thefabrication of semiconductor materials such as silicon. Moreparticularly, the present disclosure relates to a method and system forcontrolling resistivity in the formation of p-type silicon ingots thatpermits the use of low-grade silicon feedstock, for fabricating siliconthat may be ultimately useful in the manufacturing of solar cells andsimilar products.

DESCRIPTION OF THE RELATED ART

The photovoltaic industry (PV) industry is growing rapidly and isresponsible for increasing industrial consumption of silicon beyond themore traditional integrated circuit (IC) applications. Today, thesilicon needs of the solar cell industry are starting to compete withthe silicon needs of the IC industry. With present manufacturingtechnologies, both IC and solar cell industries require a refined,purified silicon feedstock as a starting material.

Materials alternatives for solar cells range from single-crystal,electronic-grade (EG) silicon to relatively dirty, metallurgical-grade(MG) silicon. EG silicon yields solar cells having efficiencies close tothe theoretical limit, but at a prohibitive price. On the other hand, MGsilicon typically fails to produce working solar cells. However, theremay be other semiconductor materials that could be useful for solar cellfabrication. In practice, however, nearly 90% of commercial solar cellsare made of crystalline silicon.

Because of the high cost and complex processing requirements ofobtaining and using highly pure silicon feedstock and the competingdemand from the IC industry, silicon needs usable for solar cells arenot likely to be satisfied by either EG, MG, or other silicon producersusing known processing techniques. As long as this unsatisfactorysituation persists, economical solar cells for large-scale electricalenergy production may not be attainable.

The resistivity is one of the most important properties of silicon (Si)used for manufacturing solar cells. That is because the solar cellefficiency sensitively depends on the resistivity. State-of-the-artsolar cell technologies require resistivity values ranging between 0.5Ωcm and 5.0 Ωcm, typically.

Besides the resistivity range, the type of conductivity is of utmostimportance when making solar cells. Conductivity needs to be eitherp-type or n-type, i.e., either electrons or holes are the majoritycurrent carriers. In current cell technologies, p-type silicon materialis typically doped with boron (B), which introduces holes or, expresseddifferently, acts as an acceptor in respective silicon. Alternatively,n-type material may be used. Such material is typically doped withphosphorus (P), which introduces electrons. Expressed differently,phosphorus is acting as a donor.

Feedstock silicon materials based on upgraded metallurgical (UM) siliconvery often contain similar amounts of B and P. As a consequence,boron-induced holes and phosphorus-induced electrons can cancel eachother, an effect called compensation. The compensation of majoritycurrent carriers often leads to a transition from p-type silicon (in thebeginning of a crystallization process) to n-type silicon (at the end ofa crystallization process). This is a consequence of differentsegregation behavior of these doping elements: phosphorus has a smallersegregation coefficient than boron. Thereby, in the case of castingingots for producing multi-crystalline (mc) Si, the process might end upwith p-type material only in the bottom and middle parts of such ingots,whereas the top part becomes n-conductive and has to be discarded.

Currently produced feedstock materials based on UMG silicon come oftenwith a base resistivity below the minimum resistivity of 0.5 Ωcm that istypically specified by solar cell manufacturers. There is a simplereason for this: expensive processes for upgrading UMG-Si are primarilyconcerned with taking out non-metals, including dopant atoms B and P. Inorder to reduce cost, there is a clear tendency to minimize suchprocessing, i.e., UMG-Si typically still contains high concentrations ofdopant atoms. As long as boron is the dominating dopant we get p-typematerial with relatively low resistivity.

Compensation of boron by phosphorus—increasing with ongoingcrystallization due to different incorporation of B and P atsolidification—results in increasing resistivity with ongoingcrystallization. So, the typically very low resistivity at the beginningof crystallization increases with ongoing crystallization. However, asalready stated, there is the general problem of too heavy resistivityincrease due to overcompensation of B by P, resulting in a transition ofconductivity from p-type to n-type. The initial addition of boron forsuppressing such a transition is not practical because one would evenfurther reduce the resistivity in bottom and middle parts of, e.g., aningot of mc-Si.

Accordingly, a need exists to control the compensation effect of thematerial, in order to increase the portion of p-type silicon material iningots thereby increasing the yield of such material.

SUMMARY

Techniques are here disclosed for providing a combination ofinterrelated steps at the ingot formation level for ultimately makingeconomically viable the fabrication of solar cells on a mass productionlevel. The present disclosure includes a method and system for formingmulti-crystalline silicon ingots, which ingots possess uniformly p-typesemiconductor material along essentially all of the ingot axial length.With the disclosed process and system, silicon ingots may be formeddirectly within a silicon melt crucible. For example, using mc-Si ingotsformed from the processes here disclosed, solar wafers and solar cellscan be made with improved performance/cost ratio, based on this mc-Simaterial.

According to one aspect of the disclosed subject matter, a semiconductoringot-forming method and system permit controlling resistivity in theformation of a silicon ingot by preparing UMG silicon feedstock forbeing melted to form a silicon melt. The present disclosure assesses theconcentrations of boron and phosphorus in said UMG-Si feedstockmaterial. Our approach of choice is analyzing the initial incorporationof B and P by in-situ measuring the resistivity in the moment when theingot formation process starts. Based on this assessment, apredetermined amount of an element of group III of the periodic system,which can be Ga, Al, a combination of Ga and Al, or another group IIIelement, is added to the UMG-Si feedstock material during thecrystallization of a large-size ingot. The predetermined quantity ofsuch group III element(s) associates with the assessed B and Pconcentrations.

The present disclosure includes melting the UMG-Si feedstock and theadded group III element(s) to form a molten silicon solution includingthe predetermined amount of group III element(s); performing adirectional solidification of the molten silicon solution for forming asilicon ingot and, by virtue of the adding a predetermined amount of thegroup III element(s), maintaining the homogeneity the resistivity of thesilicon ingot throughout the silicon ingot. Below certain resistivity itbecomes advantageous to add P or other group V elements to the group IIIelement(s). This way the useful resistivity range of reasonably yieldingp-type ingots made of compensated UMG-Si feedstock can be extended.

In one embodiment, the present disclosure also includes methods forrepeatedly measuring the dopant concentrations in the silicon meltduring ingot formation, by testing resistivity in a sample. The dopantconcentrations may then be repeatedly adjusted by adding discreteamounts of dopant in order to keep the resistivity of the ingot within apredetermined range.

In another embodiment, dopant concentrations in the silicon melt may becontinuously adjusted by a flow of dopant, rather than repeatedlytweaked by discrete additions of dopant. In this embodiment, therepeated resistivity measurements may be fed into a resistivity modelthat outputs the required flow rate of a group III or group V element,which may in turn be fed into the melt.

These and other advantages of the disclosed subject matter, as well asadditional novel features, will be apparent from the descriptionprovided herein. The intent of this summary is not to be a comprehensivedescription of the claimed subject matter, but rather to provide a shortoverview of some of the subject matter's functionality. Other systems,methods, features and advantages here provided will become apparent toone with skill in the art upon examination of the following figures anddetailed description. It is intended that all such additional systems,methods, features and advantages be included within this description, bewithin the scope of the accompanying claims.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The features, nature, and advantages of the disclosed subject matter maybecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 demonstrates one aspect of forming a silicon ingot that thepresent disclosure addresses;

FIG. 2 illustrates conceptually an embodiment of the present disclosurefor forming a silicon ingot possessing essentially all p-type siliconmaterial;

FIG. 3 shows conceptually an alternative embodiment of the presentdisclosure for forming a silicon ingot possessing essentially all p-typesilicon material;

FIG. 4 depicts the axial carrier concentration of an ingot made fromcompensated UMG silicon feedstock material;

FIG. 5 depicts the axial carrier concentration of an ingot made fromdifferently compensated UMG-Si feedstock material;

FIG. 6 depicts the axial carrier concentration of an ingot made fromanother compensated UMG-Si feedstock material;

FIG. 7 demonstrates one aspect of forming a silicon ingot using repeatedadditions of dopant that the present disclosure addresses;

FIG. 8 depicts the axial resistivity profile of an ingot made usingrepeated additions of dopant;

FIG. 9 demonstrates one aspect of forming a silicon ingot usingcontinuous flow of dopant that the present disclosure addresses;

FIG. 10 depicts the axial resistivity profile of an ingot made usingcontinuous flow of dopant;

FIG. 11 demonstrates embodiments of the present disclosure for formingboth p-type and n-type semiconductor ingots; and

FIG. 12 shows an embodiment of a continuous doping system in accordancewith the present disclosure.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The method and system of the present disclosure provide a semiconductoringot formation process for producing a multi-crystalline silicon ingot.As a result of using the disclosed subject matter, an improvement in theproperties of low-grade semiconductor materials, such as upgraded,partially purified metallurgical silicon occurs. Such improvementallows, for example, the use of UMG-Si in producing solar cells as maybe used in solar power generation and related uses. The method andsystem of the present disclosure, moreover, particularly benefits theformation of solar cells using UMG or other non-electronic gradefeedstock silicon materials, but can be used for electronic gradefeedstock silicon too.

Directional solidification (DS) leads to characteristic axialdistribution of impurities (I), controlled by impurity-specificsegregation characteristics. The so-called segregation coefficient Sthat describes segregation characteristics is described by the followingsimplified expression:

S(I)=f(C(I)_(crystal) /C(I)_(melt))

where,

-   -   S(I) represents the segregation coefficient of a specific        impurity like dopant atoms, for example I=B or I=P;    -   C(I)_(crystal) represents the concentration of, e.g., B or P in        the solidified, crystalline silicon; and    -   C(I)_(melt) represents the concentration of, e.g., B or P in the        silicon melt.        If both boron and phosphorus are present in feedstock material        used for DS crystallization of silicon, from the variation of        axial resistivity after crystallization it is possible to        conclude on the concentration ratio B/P in respective feedstock        silicon material. Conveniently, a small (e.g. from a charge size        of only a few hundred grams of feedstock material) ingot or        crystal is sufficient to get reliable axial resistivity        profiles, as long as the feedstock sampling reflects        characteristic feedstock composition with respect to the        concentration of B and P. Most important is the assessment of        the initial incorporation of B and P in such a small reference        ingot to find the right amount of dopants added in the beginning        of the process, but with the present disclosure even this is not        necessary anymore. However, we have applied this methodology for        investigating feedstock materials with different B/P ratios.

At B/P ratios close to 2 and below, we have seen a transition fromp-conductivity to n-conductivity in respective ingots, leading to yieldloss of usable p-type material of at least 10%. Typically, the portionof n-type material and the corresponding yield loss of p-type materialcan be up to almost 50%. The methods of the present disclosure maysignificantly improve the distribution of axial resistivity in ingotsmade from feedstock material with such compensation-related transitionof conductivity. These methods allow the complete suppression of thetransition to n-type material, so that up to 100% of respective ingotscan be used. More typical is a yield of approximately 95% of usablep-type material.

FIG. 1 shows diagram 10 which conceptually portrays the concentration ofboron and phosphorus appearing in a silicon feedstock melt for forming asilicon ingot. In diagram 10, ordinate 12 relates to dopantconcentration in the silicon melt, while axis 14 represents the stage ofsilicon ingot formation ranging from 0% (0.0) to 100% (1.0). Line 16represents the change in boron concentration in the silicon melt, whileline 18 represents the change in phosphorus concentration.Initially—close to 0% of ingot formation—B/P ratios >1 might exist.

As conceptual diagram 10 shows, because of the different segregationcoefficients of boron and phosphorus, at some point in the silicon ingotformation, the concentration of phosphorus exceeds the concentration ofboron. Thus, a silicon melt that began as a p-type semiconductormaterial will become an n-type semiconductor material. This is shown atline 20. Line 20, therefore, depicts that since the silicon becomes ann-type semiconductor, the resulting silicon ingot exhibits a p-njunction.

FIG. 2 depicts one embodiment of the process 30 of the presentdisclosure, wherein compensated p-type UMG silicon 32 requires apredetermined amount of a group III element 34, specifically aluminum orgallium, to form molten silicon 36. If the initial resistivity, ρ, ofUMG silicon 32 ranges between 0.15 Ωcm and 0.5 Ωcm, optionally alsophosphorus 37 may be added to a group III element of the above kind.Using a silicon ingot formation process, for example a typical castingprocess, according to the present disclosure such a process yields a100% p-type silicon ingot 38.

FIG. 3 presents an alternative embodiment of the process 40 of thepresent disclosure, wherein compensated UMG p-type silicon 42 has aninitial resistivity close to the minimum resistivity of ≈0.15 Ωcm of ausable silicon ingot 50. When approaching such a low resistivity, forexample 0.2 Ωcm, adding only group III element 44 to the silicon melt 48is not sufficient. In this case, adding additional phosphorus becomesmandatory in order to end up with 100% p-type ingot 50 in conjunctionwith a high portion of usable material out of this ingot, typicallyclose to 95% of the total ingot.

In the following FIGS. 4 through 6 and accompanying text appear axialresistivity plots and description of the resulting silicon ingots fromthe processes of FIGS. 2 and 3. FIG. 4 depicts the axial carrierconcentration of an ingot made from compensated UMG silicon feedstockmaterial with B concentration 5.0×10¹⁶ cm⁻³ and P concentration 2.4×10¹⁶cm⁻³ (concentration ratio B/P is close to 2). At the beginning ofcrystallization, i.e. at the bottom of such an ingot, the resistivity is0.6 Ωcm which corresponds to an initial majority carrier concentrationof 2.6×10¹⁶ cm⁻³. Presented is ACCEPTOR CONCENTRATION minus DONORCONCENTRATION along INGOT HEIGHT, with g being the solidified fractionof an ingot: g=0 means bottom of ingot, g=1 means top of ingot.

FIG. 4, therefore, presents a plot 60 of axial resistivity of a siliconingot when using UMG silicon feedstock material. In this case we look atmaterials that lead to an initial resistivity >0.5 Ωcm. Along ordinate62 we have resistivity-controlling numbers of electron donors, N_(d),minus numbers of electron acceptors, N_(a), as appearing in such asilicon ingot (ranging from 1×10¹⁵ to 1×10¹⁷ per cm⁻³). Along abscissa64 appears a measure of ingot formation completeness, ranging from 0.0to 1.0, where 1.0 indicates the complete ingot formation. With aninitial resistivity of the silicon ingot of 0.6 Ωcm, line 66 shows thatwithout the disclosed process, at approximately 0.8 completeness N_(d)essentially equals N_(a). At this point, the amount of phosphorus in thesilicon melt balances the amount of boron. The result of this balancebecomes first a p-n junction and then, for the remainder of the siliconingot, an n-type semiconductor material. Using this ingot would mean atleast 20% yield loss of usable p-type material.

In contrast to line 66, line 68 demonstrates the effect of adding anamount of gallium sufficient to counteract the effect of the increasingconcentration of phosphorus relative to boron (as shown in FIG. 1). Line68 shows a slight decrease in resistivity in the silicon ingot due tothe addition of gallium. However, the addition of gallium furtherprovides the beneficial effect of maintaining the difference betweenN_(a) and N_(d) almost constant throughout a large portion of thesilicon ingot. Thus, essentially until the ingot is approximately at 95%of completion, resistivity ranges between 0.53 Ωcm and 0.76 Ωcm, thesilicon remains as p-type semiconductor material, and the p-n junctionis, thereby, completely avoided. For the specific feedstock silicon inquestion, the yield loss on the ingot level is reduced by at least 15%absolute, from at least 20% in the state-of-the-art process toapproximately 5% in the disclosed process. Another essential advantageis the very tight resistivity range achieved within 95% of usable ingot.

FIG. 5 depicts the axial carrier concentration of an ingot made fromdifferently compensated UMG-Si feedstock material with B concentration7.6×10¹⁶ cm⁻³ and P concentration 5.0×10¹⁶ cm⁻³ (concentration ratio B/Pis close to 1.5). At the beginning of crystallization, i.e. at thebottom of such an ingot, the resistivity is again 0.6 Ωcm whichcorresponds to an initial majority carrier concentration of 2.6×10¹⁶cm⁻³. Presented is ACCEPTOR CONCENTRATION minus DONOR CONCENTRATIONalong INGOT HEIGHT, with g being the solidified fraction of an ingot:g=0 means bottom of ingot, g=1 means top of ingot. FIG. 5, therefore,presents a plot 70 of axial resistivity of a silicon ingot also using anUMG silicon material, as shown in FIG. 2. Ordinate 72 for the values ofN_(a)-N_(d) ranges from 1×10¹⁵ to 1×10¹⁷ per cm⁻³. Along abscissa 74ingot formation completeness ranges from 0.0 to 1.0. With an initialresistivity of 0.6 Ωcm of the silicon ingot, line 76 shows that withoutthe disclosed process, at approximately 0.6 completeness, N_(d)essentially equals N_(a). At this point, the amount of phosphorus in thesilicon melt balances the amount of boron. The result of this balancebecomes first a p-n junction and then, for the remainder of the siliconingot, an n-type semiconductor material. Using this ingot would mean atleast 40% yield loss of usable p-type material.

In contrast to line 76, line 78 demonstrates the effect of adding anamount of gallium sufficient to counteract the effect of the increasingconcentration of phosphorus relative to boron. Line 78 shows a slightdecrease in resistivity in the silicon ingot due to the addition ofgallium. However, the addition of gallium further provides thebeneficial effect of essentially reducing the difference between N_(a)and N_(d) throughout the almost entire silicon ingot formation. Thus,essentially until the ingot is approximately at the 0.95 completionpoint, resistivity ranges between 0.43 Ωcm and 0.98 Ωcm and so thesilicon remains usable p-type semiconductor material. The p-n junctionis completely avoided.

FIG. 6 depicts the axial carrier concentration of an ingot made fromanother compensated UMG-Si feedstock material with B concentration1.86×10¹⁷ cm⁻³ and P concentration 9.0×10¹⁶ cm⁻³ (concentration ratioB/P is close to 2). At the beginning of crystallization, i.e. at thebottom of such an ingot, the resistivity is only 0.2 Ωcm whichcorresponds to an initial majority carrier concentration of 9.6×10¹⁶cm⁻³. Presented is ACCEPTOR CONCENTRATION minus DONOR CONCENTRATIONalong INGOT HEIGHT, with g being the solidified fraction of an ingot:g=0 means bottom of ingot, g=1 means top of ingot. The p-n junction iscompletely avoided, and resistivity variation is still very low within95% of the total usable ingot. Thus, for the specific feedstock siliconin question, the yield loss on the ingot level is reduced by at least35% absolute, from at least 40% in the state-of-the-art process toapproximately 5% in the disclosed process.

FIG. 6 presents a plot 80 of axial resistivity of a silicon ingot todemonstrate how the present disclosure may yet beneficially affect theresistivity of feedstock material at the edge of the useful resistivityrange, as FIG. 3 depicts. Such a process may use a UMG silicon feedstockmaterial that, while demonstrating a less desirable resistivity of,e.g., approximately 0.2 Ωcm, possesses the highly desirable feature ofsignificantly lower manufacturing costs.

In plot 80, ordinate 82 for the values of N_(d)-N_(a) ranges from 1×10¹⁵to 5×10¹⁷ per cm⁻³. Along abscissa 84 ingot formation completenessranges from 0.0 to 1.0. With an initial resistivity of the silicon ingotof 0.2 Ωcm, line 86 shows that without the disclosed process, atapproximately 0.8 completeness, N_(d) essentially equals N_(a). Line 90demonstrates the effect of adding an amount of gallium sufficient tocounteract the effect of the increasing concentration of phosphorusrelative to boron. However, almost throughout the whole ingot theresulting resistivity is below the already initially very lowresistivity of 0.2 Ωcm (resistivity below this value is less useful).Thus, when using feedstock silicon leading to resistivities at the lowend of the useful range, with gallium (or aluminum) alone it ispractically impossible to bring the material into a more usefulresistivity range, even though the p-n junction is still completelysuppressed.

Line 88, however, shows a different result. Line 88 shows the result ofalso adding a certain amount of phosphorus to the feedstock silicon, inaddition to the already added gallium or aluminum. As line 88 shows, theeffect is to initially increase the resistivity and avoid the p-njunction. Thus, the silicon remains as p-type semiconductor material,and within approximately 95% of the ingot the resistivity ranges from0.17 Ωcm to 1.42 Ωcm. Only a very small percentage of this material isin the less useful range <0.2 Ωcm, as opposed to the case of adding onlythe group III element Ga (or, similarly, Al).

Summarizing, at relatively high ingot resistivity (beyond ≈0.4 Ωcm) anaddition of only aluminum or gallium can advantageously counteractcompensation of B due to P. These elements of group III of the periodicsystem have to be added to the feedstock silicon before melt-down forstarting crystallization. Contrary to the case of adding boron, whenadding Al or Ga an excellent homogenization of the resistivity along thecrystallization axis is obtained, in conjunction with avoiding strongresistivity reduction in the early stage of crystallization (whichhappens if simply adding B instead of Al or Ga). A mixture of Al and Gais also possible.

At relatively low ingot resistivity (below ≈0.4 Ωcm) one can startadding a combination of Ga and P or, alternatively, of Al and P toadvantageously counteract compensation. At very low resistivity(approaching ≈0.2 Ωcm) such a combination of a group III element and Pbecomes mandatory. Applying a certain Ga/P ratio or, alternatively, acertain Al/P ratio (whereby Ga can be partially substituted by Al, andvice versa) can now be exploited to make use of feedstock material withvery low resistivity, down to a minimum resistivity of approximately0.15 Ωcm. Such low-grade material is associated with low productioncost.

The present disclosure provides methods for controlling resistivity atgrowing silicon ingots from compensated feedstock silicon material,comprising, in one embodiment, the steps of:

-   -   assessing the concentration of boron and phosphorus that will be        initially incorporated into a specific ingot made from        compensated feedstock silicon material,    -   determining an appropriate amount of Ga and/or Al (for        relatively high resistivity to be expected from above        assessment) or, alternatively, determining an appropriate amount        of Ga and/or Al and an additional amount of P (for relatively        low resistivity to be expected from above assessment),    -   preparing respective feedstock silicon material for being melted        to form a silicon melt, by adding predetermined amounts of Ga        and/or Al (and likewise P in case of relatively low resistivity        to be expected from above assessment),    -   melting and then solidifying the above mixture of feedstock        silicon and balanced amounts of Ga and/or Al (and likewise P in        case of relatively low resistivity to be expected from above        assessment)    -   by virtue of the adding of the predetermined amount of Ga and/or        Al (and likewise P in case of relatively low resistivity to be        expected from above assessment) maintaining the homogeneity of        the resistivity of the specific silicon ingot throughout the        respective ingot.

FIG. 7 presents a plot 100 of axial dopant concentrations in a siliconingot to demonstrate how the present disclosure may provide a moretightly controlled difference between acceptor and donor dopant atomconcentrations.

In plot 100, ordinate 102 for the values of N_(a) and N_(d) ranges from1×10¹⁶ to 1×10¹⁸ per cm⁻³. Along abscissa 104 ingot formationcompleteness ranges from 0.0 to 1.0. Line 106 shows N_(a), and line 108shows N_(d).

As shown in plot 100, the concentration of acceptor atoms in the siliconmelt may be increased stepwise at intervals during ingot formation. Thisresult may be achieved by measuring the dopant concentration of theremaining molten silicon at intervals during crystallization, andthereby inferring the necessary amount of dopant to be added. One way ofmeasuring the molten silicon's dopant concentration is to insert a diprod (e.g., one made of quartz) into the molten silicon, allowing aportion of the molten silicon to cling to the dip rod and then solidify;the resistivity of this solidified piece of silicon may be measured onceit has cooled, and the relative concentration of dopant atoms may beinferred in known ways. The resistivity measurement of the solidifiedsilicon may be accomplished by either contact or contactless (e.g.,inductive) measuring. The resistivity measurement may be calibrated bycomparing it against samples having known resistivities, in order toeliminate inaccuracies from the measurement.

This procedure may allow all or most of the ingot to be formed as p-typesilicon, while also allowing the resistivity to be more tightlycontrolled, and maintained between predetermined levels. In the ingotshown in plot 100, for example, the transition to n-type silicon issuppressed until approximately 91% formation, shown by line 105. Therelative concentration of donor and acceptor atoms is kept reasonablytightly controlled up until approximately the same point, such that, byfar, most of the silicon ingot is p-type, with a resistivity in a usablerange.

FIG. 8 presents plot 120, showing the axial resistivity profilecorresponding to plot 100 in FIG. 7. In plot 120, ordinate 122 for thevalues of resistivity in Ωcm ranges from 0 to 5. Along abscissa 124ingot formation completeness ranges from 0.0 to 1.0. Line 126 shows theresistivity of the ingot from plot 100. As shown, the resistivity ofthat ingot may be maintained in a range of approximately 0.8 to 2.5 Ωcmfor approximately 90% of the ingot. In some cases, the resistivity rangemay be approximately 0.3 to 4 Ωcm. In some other cases, the resistivityrange may be 0.15 to 5 Ωcm.

However, in some cases it may be undesirable for an ingot to have anaxial resistivity profile that varies discontinuously or almostdiscontinuously, such as the profile shown in plot 120. To avoid suchdiscontinuities, the present disclosure provides methods as follows forcontinuously varying the amount of dopant flowing into the moltensilicon.

FIG. 9 presents a plot 140 of axial dopant concentrations in a siliconingot, both with and without the continuous-flow doping of the presentdisclosure. Plot 140 presents ordinate 142 for the values of N_(a) andN_(d), which ranges from 1×10¹⁴ to 1×10¹⁹ per cm⁻³. Along abscissa 144ingot formation completeness ranges from 0.0 to 1.0.

Line 146 shows the donor concentration as it increases due to theless-than-one segregation coefficient of phosphorus. Line 148 shows theacceptor concentration, which increases both because of theless-than-one segregation coefficient of the acceptor dopants, and alsobecause new dopant atoms are continuously being fed into the moltensilicon. Line 150 shows what would be the net dopant concentration inthe ingot if no additional dopant were added to the melt during ingotformation; line 150 assumes that the initial concentrations of donor andacceptor atoms are the same as in lines 146 and 148 (i.e. shown by theintersections of lines 146 and 148 with ordinate 142), respectively.Line 151 marks the p-n junction and transition to n-type material thatwould occur in line 150 if no additional dopant were added to the meltduring ingot formation. Line 152, by contrast, shows the relatively flatnet dopant concentration that may be achieved by the continuous-flowmethods of the present disclosure.

FIG. 10 presents plot 160 showing the axial resistivity profile thatcorresponds to line 152 in the continuously doped ingot from plot 140.As shown, resistivity 162 may be maintained relatively constantthroughout substantially all of ingot formation. More typically,resistivity may be maintained relatively constant through at least 90%of ingot formation. In some cases, resistivity may be maintained in therange of approximately 1.0 to 1.5 Ωcm.

FIG. 11 shows two enlarged views from plots of the type shown in plot100. View 170 is similar to the graph shown in plot 100, with the moltensilicon successively doped with acceptor atoms to form a p-type ingotwith resistivity constrained to a predetermined range.

View 172 shows another embodiment of the present disclosure, with ahigher concentration of donor atoms than acceptor atoms. This view showsthat the present disclosure may be adapted to producing n-type siliconwith resistivity constrained to a predetermined range, simply bychanging the initial concentrations of dopant atoms, and thensuccessively adding acceptor dopant in the correct quantities to keepresistivity to a desired range. N-type silicon may be desirable in somecircumstances, but it may need to have resistivity in a certain range inorder to be useful. The present disclosure provides a simple way ofproducing low-cost n-type silicon with resistivity constrained to apredetermined range. One of ordinary skill will recognize that thecontinuous-flow methods of the present disclosure may also be adapted toproducing n-type silicon with a relatively flat axial resistivityprofile.

FIG. 12 shows an embodiment of a continuous-flow doping setup.Compensated UMG silicon is placed into crucible 180 and melted. At thestage shown in FIG. 12, recrystallization has already begun. Siliconingot 181 is shown at the bottom of crucible 180, underneath moltensilicon 182. At intervals during recrystallization, dip probe 183 isinserted into molten silicon 182 to remove a portion for testing. Theremoved portion of molten silicon (not shown) is then solidified outsidethe crucible and its resistivity is measured at step 184. The measuredresistivity is then fed into a computer model of the process at step185, which reveals the relative dopant concentrations and the necessarydoping adjustment. The results of that calculation are fed into a feedercontrol system at step 186, which then adjusts the flow rate of dopantentering the melt through feeder 187. The dopant flowing into crucible180 may be a powdered doped silicon, or any other type of dopant thatmay continuously flow. Using doped silicon powder may allow greatercontrol over the dopant level in molten silicon 182 than using, e.g.,pure dopant.

The semiconductor processing features and functions described hereinprovide for resistivity control in the formation of p-type and n-typesemiconductor ingots. Although various embodiments which incorporate theteachings of the present disclosure have been shown and described indetail herein, those skilled in the art may readily devise many othervaried embodiments that still incorporate these teachings. The foregoingdescription of the specific embodiments, therefore, is provided toenable a person skilled in the art to make or use the claimed subjectmatter. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinnovative faculty. Thus, the claimed subject matter is not intended tobe limited to the embodiments shown herein, but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for controlling resistivity in theformation of a silicon ingot, comprising the steps of: preparing acompensated, upgraded metallurgical silicon feedstock for being meltedto form a silicon melt, said compensated, upgraded metallurgical siliconfeedstock comprising a predominantly p-type semiconductor; assessingquantitatively the relative concentrations of boron and phosphorus insaid compensated, upgraded metallurgical silicon feedstock; adding tosaid compensated, upgraded metallurgical silicon feedstock a firstgreater-than-zero quantity of a first element from the group consistingof boron, aluminum, gallium, mixtures of boron, aluminum and gallium,other Group III elements, phosphorus, or other Group V elements, whereinsaid quantitatively assessed relative concentrations of boron andphosphorus determine said first greater-than-zero quantity; melting saidupgraded metallurgical silicon feedstock and said firstgreater-than-zero quantity of said first element to form a moltensilicon solution including said first greater-than-zero quantity of saidfirst element; periodically performing resistivity measurements on saidmolten silicon solution; adding to said molten silicon solution a secondgreater-than-zero quantity of a second element from the group consistingof boron, aluminum, gallium, mixtures of boron, aluminum and gallium,other Group III elements, phosphorus, or other Group V elements, whereinsaid resistivity determines said greater-than-zero quantity; andperforming a directional solidification of said molten silicon solutionfor forming a silicon ingot and, by virtue of said adding said firstgreater-than-zero quantity of said first element and said secondgreater-than-zero quantity of said second element suppressing thetransition of said silicon ingot to n-type material by virtue ofreducing the effect of differing boron and phosphorus segregationcoefficients.